Quenching-partitioning steel, i.e., Q&P steel, is a research focus in the field of high strength steel in the past decade, which aims most importantly for improving the strength and plasticity of the steel simultaneously, that is, for improving the product of strength and plasticity of the steel. Currently, it is generally recognized that Q&P steel is an important new steel among the third generation of advanced high-strength steel in the field of automotive steel.
The primary processes of Q&P steel are: heating the steel to completely austenitic area or partially austenitic area; after performing homogenization treatment for a period of time, quenching rapidly to a temperature between Ms and Mf (Ms and Mf indicates respectively the start and end temperatures of the martensite transformation), so as to obtain the martensite plus retained austenite structure with a certain amount of retained austenite structure; subsequently preserving heat at the cease cooling temperature of the quenching or a temperature slightly higher than the cease cooling temperature for a period of time, so as to spread the carbon atom from the oversaturated martensite into the retained austenite, thus stabilizing the retained austenite; then quenching again to the room temperature.
The initial research and application of Q&P steel focused on the demand of the automobile industry on the high-strength and high-plasticity steel. It is not difficult to see from the processes of Q&P steel that its process line is complicated, and after the steel sheet is subjected to the first quenching, it needs to be rapidly heated up to a temperature and kept for a period of time. This two-stage Q&P process is difficult to be implemented for the hot rolling manufacturing process, but gives a good reference for manufacturing the hot rolling high-strength steel. During the hot rolling, one-stage Q&P process can be used, that is, after the finish rolling, the steel is reeled subsequent to being online quenched to a certain temperature below Ms. The typical structures of Q&P steel are martensite plus a certain amount of retained austenite, thereby presenting high strength and good plasticity.
China patent CN102226248A discloses a C—Si—Mn hot rolling Q&P steel, but with respect to the design of alloy element, no micro-Ti treatment is carried out; China patent CN101775470A discloses a manufacturing process of the complex-phase Q&P steel, which is actually a two-stage process of manufacturing Q&P steel; China patent CN101487096A discloses a C—Mn—Al Q&P steel, which features chiefly in high elongation rate but low strength.
The above patents use the heat treatments and control easily the volume fraction of ferrite through heating in two phase areas; but for the continuous hot rolling, the heating temperature is generally in the complete austenite area and the finishing temperature is generally above 780° C., while the start precipitating temperature of ferrite is mostly below 700° C. Consequently, it is difficult to implement in the actual hot rolling that ferrite is obtained by lowering the finish rolling temperature.